Conjugated Polymers Containing Sulfonic Acid Fluorene Unit for

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C: Energy Conversion and Storage; Energy and Charge Transport

Conjugated Polymers Containing Sulfonic Acid Fluorene Unit for Achieving Multiple Interfacial Modifications in Fullerene-Free Organic Solar Cells Lili Lu, Qian Kang, Chenyi Yang, Bowei Xu, and Jianhui Hou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04093 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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The Journal of Physical Chemistry

Conjugated Polymers Containing Sulfonic Acid Fluorene Unit for Achieving Multiple Interfacial Modifications in Fullerene-free Organic Solar Cells Lili Lu, Qian Kang, Chenyi Yang, Bowei Xu*, Jianhui Hou* State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Conjugated polymers (CPs) as interfacial layer materials have played crucial role in improving the power conversion efficiency (PCE) of organic solar cells (OSCs). Sulfonic acid-functionalized fluorene units have showed promising applications in developing CP interlayers. However, the development of CPs containing sulfonic acidmodified fluorene unit into interlayers with multiple functions still remain a desired objective. In this work, three CPs named PFS, PFSF, and PFB are designed and synthesized based on the sulfonic acid fluorene unit as interlayer materials for fullerenefree OSCs. The energy level of CPs is effectively tuned by copolymerizing sulfonic acid fluorene with different units including bithiophene, difluorinated bithiophene and benzene. The as-synthesized PFS can be used as the anode interfacial layer (AIL) and the corresponding OSC exhibits a PCE of 11.0%. Meanwhile, PFSF and PFB can serve as the cathode interfacial layer (CIL). More importantly, PFS can be used as bifunctional interlayer for simultaneously modifying both the anode and the cathode in the same device. The fullerene-free OSC with symmetric structure is fabricated by using PFS as both the AIL and the CIL, attaining a PCE of 9.54%. Furthermore, by using PFS as AIL and PFSF as CIL, an efficient OSC is fabricated with a PCE of 10.2%.

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1. INTRODUCTION Fullerene-free organic solar cells (OSCs), in which a blend of a polymer donor and a non-fullerene small molecule acceptor is used as the photoactive layer, has been deemed as a promising energy-conversion technique due to its excellent photovoltaic performance and potential in realizing low-cost solvent manufacturing1-5. Benefiting from the rational molecular design of photoactive and interfacial layer materials, power convention efficiencies (PCEs) of fullerene-free OSCs has been boosted to over 12%, indicating a bright future for their practical applications6-9. In recent years, interfacial layers between the active layer and the electrode play a vital role in enhancing the PCEs of OSCs by facilitating hole and electron extraction to the respective electrodes10-12. There are mainly two kinds of interlayers in OSCs: anode interfacial layer (AIL) for hole extraction and cathode interfacial layer (CIL) for electron extraction13-14. Currently, developing new interfacial layer materials has become one of the most effective ways to improve the photovoltaic performance of OSCs. Conjugated polymers (CPs) consisting of π-conjugated backbones with polar functional groups have been developed as interfacial layer materials owing to their good solution-processing ability and adjustable photoelectric properties15. One remarkable advantage of the CPs as the interlayer materials is that their molecular structures can be readily modified through versatile synthetic chemistry, which offers opportunities for tuning the chemical, optical and electrical properties of the materials16-18. Using CPs as the interfacial layers, the work functions (WF) of electrodes can be effectively tuned, 3



making them capable of serving as anode or cathode in OSCs. For example, Bazan et al. designed and synthesized a narrow-band-gap conjugated polymer CPE-K and fabricated an OSC with a PCE of 8.0% using the CPE-K as the AIL19. Moreover, other CPs, such as PCP-Na20, P121, PhNa-1T22 and p-PFP-HD23, that can tune the WF of electrode to 4.9-5.4 eV are also developed for serving as AILs. As for CILs, a series of CPs functionalized with amino group are developed as CIL materials, such as PFN24-25, PNDIT-F3N-Br 26and their analogues. The polar groups in the side-chains of CPs can form a thin layer of permanent interfacial dipole at the active layer/cathode interface, which is favorable to the electron extraction. Moreover, Hou’s group reported an interesting CP, namely PFS27, which can be used as both AIL and CIL in one OSC device. However, although the newly developed CP interlayer materials have greatly improved the OSCs efficiency, most of the studies lie in fullerene-based OSCs. Considering the high PCE of fullerene-free devices, developing new CPs interlayer materials and exploring their applications in fullerene-free OSCs is of great significance.28-30 Recently, Zhang et al. synthesized a series of PFS derivatives, PFS3C, PFS-4C and PFS-6C, based on the fluorene unit modified with different alkyl sulfonic acid pendants31. PFS-4C was used as the AIL to modify the fullerene-free OSC, and a PCE of 10.5% was achieved. The above results suggest that the sulfonic acidmodified fluorene unit is a promising candidate in developing superior CP interlayer materials. However, these PFS derivatives can only serve as AIL in fullerene-free OSCs, and thus the development of CPs containing sulfonic acid-modified fluorene unit into interlayers with multiple functions has become a desired objective. 4


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Herein, we designed and synthesized three CPs, namely PFS, PFSF and PFB, to be used as AIL and CIL for fullerene-free OSCs. By the copolymerization of sulfonic acid-modified fluorene with different units, including bithiophene, difluorinated bithiophene and benzene, the energy levels of the three CPs could effectively be tuned, so that their multiple functions of modifying fullerene-free OSCs could be developed. In this work, PFS works well as AIL in PBDB-T: IT-M-based32-33 OSC and the PCE of the PFS-modified device is 11.0%, which is comparable to that of the PEDOT: PSS3436

devices. Differing from PFS, both PFSF and PFB can be used as CILs in fullerene-

free OSCs. Furthermore, we investigated the bifunctional modification of the three CPs by fabricating OSCs with symmetrical device structures, in which the single polymer material could be employed simultaneously as the AIL and the CIL in the same device. Interestingly, the OSC device with PFS as both the anode and cathode interlayers can work well, and a PCE of 9.54% was achieved. Finally, by using PFS as AIL and PFSF as CIL, an efficient OSC with a PCE of 10.2% is fabricated, indicating a promising application of sulfonic acid fluorene-based CPs as interlayers with multiple functions in fullerene-free OSCs.

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2. EXPERIMENTAL SECTION 2.1 Materials and Synthesis. Monomer 1, 3, 4 and 5 were purchased from Solarmer Materials, Inc. Monomer 2 was prepared through a reported method. Pd (PPh3)4 was purchased from Frontier Scientific Inc. Other reagents were common commercial level and used without further purification. All reactions were carried out under an argon atmosphere. The polymers of PFS, PFSF and PFB were prepared as follows. PFS. In a 50 mL flask of flame-drying, monomer 2(0.6099g, 1.0mmol) and monomer 3 (0.4939g, 1.0mmol) were dissolved in 6 mL dry DMF. After being flushed with argon for 5 min, Pd (PPh3)4 (0.015 g, 0.013mmol) was added. The reaction mixture was degassed with argon for another 15 min and then was vigorously stirred at 95 °C for 48h. After the reaction finished, the polymer was poured into 500mL 1M hydrochloric acid and the mixture was stirred for 0.5 h at room temperature. Afterwards, the polymer was precipitated and filtered in a Buchner funnel. The polymer was purified by precipitation in ethyl ether several times with a yield of 43%. 1H-NMR (DMSO, 400 MHz, δ): 8.42−7.41 (m, br, 10H), 2.51−1.90 (m, br, 8H), 0.86 (m, br, 4H); Anal. Calcd for [C27H26O6S4]: C, 56.42; H, 4.56; Found: C, 56.01; H, 4.48; GPC: Mn =9855, Mw =15107, PDI =1.53. PFSF. The synthesis procedure of PFSF is same to that of PFS, but now monomer 2(0.6099g, 1.0 mmol), monomer 4 (0.5279g, 1.0 mmol), Pd (PPh3)4 (0.015 g, 0.013 6


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mmol) were added in 6 mL dry DMF. Yield: 38%.1H-NMR (DMSO, 400 MHz, δ): 8.42−7.41 (m, br, 8H), 2.49−1.90 (m, br, 8H), 0.82 (m, br, 4H).Anal. Calcd for [C27H24O6F2S4]: C, 53.10; H, 3.96; Found: C, 51.43; H, 3.86; GPC: Mn =17143, Mw =92873, PDI =5.4. PFB. Monomer 2 (0.6099g, 1.0 mmol), monomer 5 (0.2178g, 1.0 mmol) and Na2CO3 (525mg, 4.99mmol) were dissolved in 6 mL dry DMF and 1.5ml H2O. After being flushed with argon for 5 min, Pd (PPh3)4 (0.015 g, 0.013 mmol) was added. Then the next operation and product purification method were same to PFS.Yield:50%. 1HNMR (DMSO, 400 MHz, δ): 8.21−7.37 (m, br, 10H), 2.50−2.15 (m, br, 8H), 0.86 (m, br, 4H); Anal. Calcd for [C25H24O6S2]: C, 61.71; H, 5.39; Found: C, 60.81; H, 5.13; GPC: Mn =11605, Mw =38059, PDI =3.3.

2.2 Device Fabrication and Characterization.

Devices A1-A3 were fabricated taking PFS, PFSF and PFB as anode interlayers (AIL). The A-type devices structure (ITO/Interlayer/PBDB-T: IT-M/PFN-Br/Al) were fabricated according to the following conditions: The pre-cleaned ITO-coated glass substrates were UV/ozone-treated for 20 min. PFS, PFSF and PFB were dissolved in methanol with various concentrations of 1mg/ml,1.5mg/ml and 2.0mg/ml, respectively. Interlayer was spin-coated at 2000/3000 rpm/min for 60s on the ITO electrode and thermal treated at 120oC for 10min. Thickness was optimized by varying the concentration of solution and revolving speed. Then 7



PBDB-T: IT-M (in a ratio 1:1, polymer concentration of 10mg/mL-1) based photoactive blend layer was spin-coated (2000rpm/min, 30s) on substrates in glovebox. The optimal thickness of the photoactive layer was determined to the solution concentration and spin-speed. Then, PFN-Br with 0.5mg/mL was spin-coated at 2000rpm/min for 30s. Finally, Al was deposited on the photoactive layer as cathode under high vacuum as published previously by our group.

Devices B1-B3 were fabricated taking PFS, PFSF and PFB as cathode interlayers (CIL). The B-type devices structure (ITO/PEDOT: PSS/PBDB-T: ITM/Interlayer/Al) were fabricated according to the following conditions: The pre-cleaned ITO-coated glass substrates were UV/ozone-treated for 20 min. PEDOT: PSS was spin-coated (4000 rpm/min, 60s) on the ITO electrode and thermal treated at 150℃ for 10min in air. The PBDB-T: IT-M photoactive layer was prepared according to the process mentioned above. Then PFS, PFSF and PFB were dissolved in methanol with various concentrations of 0.1mg/ml, 0.2mg/ml and 0.4mg/ml, respectively. Interlayers were spin-coated at 3000 rpm/min for 30s on the photoactive layer and thermal treated at 120oC for 10 min. Finally, Al electrode was deposited on the substrate as cathode under high vacuum.

Devices C1-C4 were fabricated taking PFS, PFSF and PFB as anode interlayers (AIL) and cathode interlayer (CIL) simultaneously. The C-type devices structure (ITO/Interlayer/PBDB-T: IT-M/Interlayer/Al) were fabricated according to the following conditions: 8


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The pre-cleaned ITO-coated glass substrates were UV/ozone-treated for 20 min. PFS, PFSF and PFB were dissolved in methanol with the optimal concentration of 1.5mg/ml and spin-coated on the ITO-electrode according to the devices A-1 mentioned above. The preparation of PBDB-T: IT-M as photoactive layer was same to the devices A-1 and A-2. Then 0.2mg/ml PFS, PFSF, PFB were prepared to be spin-coated (3000rpm/min, 30s) on the photoactive layer. At last, Al was evaporated on the substrate as cathode under high vacuum. 2.3 Instruments and Measurements 1

H NMR spectra was recorded on a Bruker advanceⅡ-400 spectrometer at 298K,

and the chemical shifts of DMSO solvent peaks is 2.50 ppm for 1H NMR. Numberaverage (Mn) and weight-average (Mw) molecular weights were determined by GPC on a Viscotek TDA 305 (Malvern, UK) instrument with N, N-Dimethylformamide (DMF) as eluent. The elemental analysis was performed on a FLASH EA1112 for C, H, and N elements. UV−vis absorption spectra were obtained by UH5300. Cyclic voltammetry (CV) measurements were performed on a CHI650D Electrochemical Workstation with a three-electrode cell in a 0.1 M anhydrous and argon-saturated solution of ferrocene/ferrocenium redox couple (Fc/Fc+) as the internal calibration in acetonitrile. Current density-voltage (J-V) characteristics were measured under 100 mW/cm2 AM1.5G light source, using a solar simulator (AAA grade). The effective area of the device is 3.7 mm2. The EQE data were measured by a Solar Cell Spectral Response Measurement System QE-R3011 (Enli Technology Co., Ltd.). 9



3. RESULTS AND DISCUSSION 3.1 Synthesis and characterizations

Scheme 1. Synthetic routes of PFS, PFSF and PFB. The chemical structures and synthetic routes of the three CPs PFS, PFSF and PFB are shown in Scheme 1. Monomer 2 was synthesized according to the literature method27. PFS, PFSF and PFB were synthesized via the Pd-catalyzed coupling reaction37-38 under argon atmosphere in a dimethylformamide (DMF)/water cosolvent. All the polymeric product were acidified by 1M hydrochloric acid and were purified through the precipitation in diethyl ether, as reported in our previous work. The yields of the three CPs are approximately 40%. Chemical structures of the three polymers were confirmed by 1H NMR and elemental analysis. The number-average molecular weight (Mn) and polydispersity of the CPs were determined by gel permeation 10


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chromatography (GPC) using N, N-dimethylformamide (DMF) as the eluent. As shown in Table 1, the Mn of PFS, PFSF and PFB are 9.86 kDa, 17.1 kDa and 11.6 kDa, respectively. All the three CPs can be easily dissolved in polar organic solvents, such as methanol, isopropanol and DMF. Notably, these polymers are insoluble in organic solvents that are widely used in making the active layers in OSCs such as chloroform, toluene, and chlorobenzene, which is crucial for the multilayer deposition in OSCs fabrication.

3.2 Photophysical Properties UV-vis absorption spectra of PFS, PFSF and PFB in solution and as solid films are shown in Figure 1a. The absorption peaks of PFS and PFSF in methanol are principally located at approximately 430nm, which is attributed to the π-π* transition of the polymer backbone39-41. Compared to PFS and PFSF, PFB exhibits a blue-shifted absorption at 364nm due to the weakened intrachain charge transfer interaction of the PFB backbone. The absorption spectra of the CPs films is obviously red-shifted compared to the absorptions of the CPs solutions, which might be ascribed to the interchain π-π stacking of polymer backbones in solid films. The band gaps (Eg) of the CPs are estimated to be 2.34 eV for PFS, 2.39 eV for PFSF and 2.93 eV for PFB from the absorption onset in the UV-vis spectra of the film sample. By comparison of PFS with PFSF, the introduction of F atoms has little influence on the Eg of the polymers, which is consistent with the results in our previous work. As shown in Figure 1b, molecular energy levels of polymers PFS, PFSF and PFB are determined by 11



electrochemical cyclic voltammetry (CV). The HOMO levels of PFS, PFSF and PFB were -5.15eV, -5.34 eV and -5.70eV, respectively, as calculated from the onset oxidation potentials. The LUMO levels of -2.80 eV for PFS, -2.95 eV for PFSF and 2.77 eV for PFB were determined via the differences between the HOMO and the optical band gap. It is revealed that both the HOMO and the LUMO levels of PFSF are reduced simultaneously by the incorporation of F atoms into the polymer backbone. The above results suggest that the energy level of the sulfonic acid fluorene-based CPs can be readily tuned by the copolymerization of the propyl sulfonic acid-modified fluorene with different units. Ultraviolet photoelectron spectroscopy (UPS) measurement was carried out to investigate the effect of the CPs on tuning the WF of ITO electrodes. The UPS spectra of bare ITO and ITO modified with PFS, PFSF and PFB are displayed in Figure 1c. Upon the modification of PFS, PFSF and PFB, the WFs of ITO electrode are tuned to 4.90 and 5.10 and 4.70 eV, respectively. Clearly, PFS and PFSF containing the fluorene-bithiophene repeat have the similar effect to PEDOT: PSS on increasing the WF of ITO anode, which is of importance for the CPs to modify ITO anode. (b) PFS-solution PFS-film PFSF-solution PFSF-film PFB-solution PFB-film

1.0 0.8 0.6 0.4

(c) 1.2 PFS PFSF PFB

Normalized Intensity

1.2

Current (a.u.)

(a) Normalized Absorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.2 0.0 300

bare ITO PFS PFSF PFB

1.0 0.8 0.6 0.4 0.2 0.0

400

500

600

Wavelength (nm)

700

800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Voltage (V)

3

4

5

6

7

Kinetic Energy (eV)

8

Figure 1. (a) UV-vis absorbance spectra of PFS, PFSF and PFB in methanol solution and as solid films. (b) Cyclic voltammograms of PFS, PFSF and PFB films in 12


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acetonitrile solution. (c) UPS spectra of bare ITO and ITO modified with PFS, PFSF and PFB (The thickness of the CPs was ca. 7 nm.). Table 1. Summary of UV-vis spectra, energy levels and Molecular Weights. Abs/sol(nm)

Abs/film(nm)

peak

onset

peak

PFS

430

494

PFSF

434

PFB

364

interlayers

LUMO

HOMO

Mn

onset

(eV)

(eV)

(kDa)

462

528

-2.80

-5.15

9.86

486

454

518

-2.95

-5.34

17.6

404

376

422

-2.77

-5.70

11.6

3.3 Performances of PFS, PFSF and PFB interlayers in OSCs To investigate the effects of PFS, PFSF and PFB as interlayers in OSCs, we fabricated two types of devices, device A and device B. PBDB-T was used as the polymer donor and IT-M was used as the non-fullerene acceptor to make the photoactive layer. The molecular structures of PBDB-T and IT-M are provided in Figure 2a. The schematic energy level diagram of the interlayer and photoactive layer materials used in this study is shown in Figure 2b. As shown in Figure 2c and 2d, the device structures are as follows: Device A: ITO/CPs (PFS, PFSF or PFB)/PBDB-T: ITM/PFN-Br/Al; Device B: ITO/PEDOT: PSS/PBDB-T: IT-M/CPs (PFS, PFSF or PFB)/Al. The device with a structure of ITO/PEDOT: PSS/PBDB-T:IT-M/PFN-Br/Al was also fabricated as control. The current density-voltage (J-V) characteristics

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measured under AM1.5G 100 mW/cm2 illumination are provided in Figure 3 and the corresponding photovoltaic parameters are summarized in Table 2 and Table 3.

Figure 2. (a) Molecular structures of PBDB-T and IT-M. (b) Device structure of devices A1-A3. (c) Device structure of devices B1-B3. (d) The energy level diagram of interlayers and active layers. Firstly, we investigate the effects of the three CPs in serving as AILs by fabricating the devices A1-A3. The thickness of the CP AILs was optimized by tuning the concentration of the CP solutions used in the spin-coating process. As shown in Table 2, the optimal solution concentration is 1.5 mg/mL for all the CPs used as AILs. The 14


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optimal thickness of the CPs used as AILs was ca. 7 nm, which was determined by using the calibration curve (Figure S1 and Table S1). The device A1 using PFS as the AIL exhibits the best photovoltaic performance with a Voc of 0.92 V, Jsc of 17.2 mA/cm2, FF of 0.70 and PCE of 11.0%, which is comparable to that of the PEDOT:PSS-modified OSC. When using PFSF as AIL, although the device A2 can work normally and the JV curve displays a typical diode characteristic, the PFSF-modified device only shows a moderate PCE of 9.09%. Both PFS and PFSF can increase the WF of ITO anode, which improve hole extraction from the photo active layer. The use of PFS and PFSF to modify ITO anode is similar to the commonly used AIL materials such as PEDOT: PSS and PCP-Na. Moreover, the superiority of PFS in modifying ITO to PFSF may be because the high-lying LUMO level of PFS can block electron transfer from the photoactive layer to the anode, resulting in the improved FF of the device A1. As reported in previous works, AIL materials with high-lying LUMO level, such as grapheme oxide42 and microporous polymer43, can provide good electron-blocking capacity in modifying anodes, leading to high FF of the device. In contrast, the device using PFB as AIL shows a poor PCE of 3.91% with a significant “S-shape” J-V curve, indicating the existence of an interfacial barrier for the hole transport. The large energetic offset between the WF of PFB-modified ITO and the HOMO level of PBDBT is unfavorable to the hole transfer from photoactive layer to anode, leading to the severe charge accumulation and recombination at the active layer/anode interface, and hence decreases the FF of the device.

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(b) 80

5 0

60

PFB/BHJ/PFN-Br PFSF/BHJ/PFN-Br PFS/BHJ/PFN-Br PEDOT:PSS/BHJ/PFN-Br

-5 -10

EQE(%)

Current Density (mA/cm2)

(a)

(c)

40 PEDOT/BHJ/PFN-Br PFS/BHJ/PFN-Br PFSF/BHJ/PFN-Br PFB/BHJ/PFN-Br

20

-15 -20 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 300

1.2

Voltage(V)

400

500

600

700

800

Wavelength(nm)

(d) 80

5

70

0

PEDOT:PSS / BHJ / PFS PEDOT:PSS / BHJ / PFSF PEDOT:PSS / BHJ / PFB

-5

EQE ()

Current Density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-10

60 50 40 30 20

-15

10

-20 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 300

1.2

PEDOT:PSS / BHJ / PFS PEDOT:PSS / BHJ / PFSF PEDOT:PSS / BHJ / PFB 400

500

600

700

800

Wavelength (nm)

Voltage(V)

Figure 3. (a) J-V and (b) EQE curves of devices A1-A3 and the PEDOT: PSS device. The CPs were deposited from the solution of 1.5 mg/mL. (c) J-V and (d) EQE curves of devices B1-B3. The CPs were deposited from the solution of 0.2 mg/mL. Table 2. Photovoltaic Parameters of the A-type devices A1-A3. concentrations Num. interlayer

of CPs (mg/mL)

A1

A2

A3

PFS

PFSF

PFB

Rsh

Rs

(Ω·cm2) (Ω·cm2)

Voc

Jsc

(V)

(mA/cm2)

FF

PCE (%)

1

11.8

364

0.871

15.2

0.58

7.66

1.5

3.89

790

0.921

17.2

0.70

11.0

2

20.3

720

0.880

16.1

0.59

8.40

1

95.9

897

0.859

16.7

0.50

7.12

1.5

12.2

887

0.905

17.2

0.58

9.09

2

1.68k

752

0.962

15.8

0.42

6.32

1

102

336

0.628

15.7

0.39

3.82

1.5

2.40k

629

0.776

16.0

0.32

3.91

16


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2 PEDOT: PSS/BHJ/PFN-Br

--

1.32k

377

0.763

15.1

0.288 3.32

5.23

1.12k

0.934

16.3

0.709 10.8

Afterwards, the performances of PFS, PFSF and PFB in modifying the cathode is evaluated by the incorporation of the three CPs into the devices B1-B3. The optimal thickness of CPs used as CILs is ca. 3 nm when spin-coating 0.2 mg/mL CPs solution on the photoactive layer (Figure S1 and Table S1). As shown in Figure 3c, all the three CPs work well as CILs and the device B2 exhibits the best photovoltaic performance with a PCE of 10.7% (Table 3), which is comparable to that of the PFN-Br-modified device. It is well recognized that the polar function groups on polymer side-chains can form the permanent interfacial dipole which facilitates the electron extraction.44 To confirm the existence of interfacial dipole at the photoactive layer/cathode interface, scanning Kelvin probe microscopy (SKPM) was performed to probe the surface potential of different layers (Figure 4). As shown in Figure 4e, the surface potentials of the CP-based CILs are 20 mV, 100 mV and 420mV more positive than that of the bare photoactive layer (Figure 4e). This indicates the existence of microscopic electric dipole moment with the positive charge end pointing toward the cathode and the negative charge end pointing toward the active layer.45 The direction of this dipole moment is favorable to the electron transfer from photoactive layer to cathode. Therefore, the thickness of CPs for modifying cathode has to be limited within 3 nm, in which case interfacial dipole is formed and works to facilitate electron extraction. Moreover, the PFSF CIL exhibited the most positive surface potentials among the three CPs, which is consistent with the observation that OSCs with the PFSF CIL showed the 17



best device performance. Besides interfacial dipole, the doping effect on fullerene derivatives observed in fullerene-based OSCs was considered as a favorable factor for electron transport and collection. Therefore, we conducted the electron spin resonance (EPR) spectroscopy to probe the doping behavior between CP-based CILs and the nonfullerene acceptor IT-M. However, no EPR signal was observed for the mixtures of CPs and IT-M (Figure S4), meaning that the doping effect on improving device performances could be excluded. Therefore, based on the above results, we attribute the improved photovoltaic performance of B-type devices to the formation of interfacial dipole by the ultrathin CPs films. Moreover, although the three CPs have the same propyl sulfonic acid group, the PFSF possesses the lowest LUMO level, which might be favorable to the electron extraction from the photoactive layer to the CIL. Therefore, the combination of the interfacial dipole formed by the sulfonic acid and the low-lying LUMO level is the possible reason for the best performance of PFSF in modifying the cathode in OSCs.

18


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Figure 4. The surface potential distribution probed by SKPM for (a) bare photoactive layer of PBDBT: ITM and photoactive layer covered with (b) PFB, (c) PFS and (d) PFSF. (e)The surface potential difference of PBDBT: ITM without and with CPs. (f) Schematic illustration of the experimental setup. Table 3. Photovoltaic Parameters of the B-type devices B1- B3. concentration Num. interlayer

of CPs (mg/mL)

B1

B2

B3

PFS

PFSF

PFB

Rs

Rsh

(Ω·cm2) (Ω·cm2)

Voc

Jsc

(V)

(mA/cm2)

FF

PCE (%)

0.1

7.32

1.06k

0.870

17.4

0.65

9.75

0.2

7.46

715

0.885

17.1

0.64

9.74

0.4

7.21

1.22k

0.869

17.0

0.65

9.55

0.1

5.12

617

0.917

17.0

0.65

10.1

0.2

5.30

1.73k

0.904

17.4

0.68

10.7

0.4

7.64

872

0.912

17.5

0.67

10.6

0.1

7.89

950

0.886

17.3

0.60

9.24

0.2

5.51

1.06k

0.907

15.8

0.68

9.80

19



0.4

6.91

954

0.893

Page 20 of 32

16.0

0.66

9.48

As reported in previous works, some special materials can serve as both the AIL and CIL in OSCs. Such bifunctional interlayers can be used to fabricated OSCs with symmetric structure, in which the AIL and CIL are made from the same material; this provides a potential approach to simplify the fabrication process of OSCs. As shown in Figure 5a, devices C1-C3 with symmetric structure of ITO/CPs (PFS, PFSF or PFB)/PBDB-T: IT-M/CPs (PFS, PFSF or PFB)/Al were fabricated to explore the application of PFS, PFSF or PFB as bifunctional interlayers for fullerene-free OSCs. For each CP, the concentrations of the polymer solutions for making AIL and CIL are 1.5 mg/mL and 0.2 mg/mL, respectively. The J-V curves and EQE spectra of the devices C1-C3 are presented in Figure 5b and 5c, and the corresponding photovoltaic parameters are summarized in Table 4. The devices C2 and C3 show the “S-shape” JV curves, meaning that PFSF and PFB cannot be used as bifunctional interlayers to construct symmetric devices. In contrast, the device C1 with PFS as the AIL and CIL works well and exhibits a high PCE of 9.54%.

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Figure 5. (a) Schematic structures of devices C1-C3. (b) J-V curves and (c) EQE spectra of devices C1-C3. (d) Schematic build-in field in the device C1. (e) Impedance spectra (EIS) and equivalent circuit models for the C-type devices. The schematic illustration of the work mechanism for device C1 is presented in Figure 5d, which interprets the mechanism of PFS in modifying both the anode and cathode in one device. PFS functions as AIL and CIL through different mechanisms, which could be realized by controlling the PFS thickness. As for the modification of ITO anode, a PFS layer of 7 nm was used to enhance the WF of ITO; this is helpful to 21



form ohmic contact at the photoactive layer/anode interface. The performance of 7 nm PFS in enhancing the WF is similar to PEDOT: PSS. While modifying Al cathode, a layer of electric dipole forms at the photoactive/Al interface by using the ultrathin PFS of 3 nm, which enhances the electron extraction (Figure 5d). To investigate the charge collection properties of C-type devices, electrochemical impedance spectroscopy (EIS) was performed to examine the interface resistance of the devices. Figure 5e shows Nyquist plots of C-type devices in light. A bias voltage equal to Voc was applied to vanish the total current. The data were fitted using the equivalent-circuit model according to the literature method. The surface resistance (Rsurface) of the devices C1, C2 and C3 was estimated to be 8.40kΩ, 22.2kΩ and 27.1kΩ, respectively. The results indicate that, by using PFS as both AIL and CIL, the Rsurface of device C1 was significantly decreased, which can reduce the charge recombination as well as improve the charge transfer. The above results indicate that PFS can be used as a bifunctional interlayer for fullerene-free OSCs.

(a)

5

(b)

PFS / BHJ / PFSF

2

Current Density (mA/cm )

80 70

0

60

-5

EQE ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

-10

50 40

PFS / BHJ / PFSF

30 20

-15

10

-20 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 300

1.2

400

500

600

700

800

Wavelength (nm)

Voltage (V)

Figure 6. (a) J-V curves and (b) EQE spectra of device C4. To further demonstrate the diverse functions of sulfonic acid fluorene-based CPs as interlayers for modifying electrodes, efficient fullerene-free device was fabricated 22


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by using PFS as the AIL and PFSF as the CIL. As shown in Figure 6, the fullerene-free OSC using PFS as the AIL and PFSF as the CIL exhibits a high PCE of 10.2%. Moreover, in order to further investigate the applicability of the CP-based interlayers, CPs were also employed to modify other efficient photoactive layers, such as J52-2F: IT-M, PBDTTTEFT: IEICO-4F and PBDB-T: ITIC. The chemical structures of these photoactive layer materials were presented in Figure S2. The J-V curves of the devices were provided in Figure S3, and the corresponding photovoltaic parameters were summarized in Table S2. The PCEs of CP-modified devices using the J52-2F: IT-M, PBDTTTEFT: IEICO-4F and PBDB-T:ITIC as photoactive layers are 10.4%, 9.61% and 9.20%, respectively. These results indicate that CP-based polymers are universal interlayers for OSCs. Based on the above results, the sulfonic acid fluorene unit is demonstrated to be a promising candidate to develop CP-based interlayers with different energy levels for application in fullerene-free OSCs. Table 4. Photovoltaic parameters of the C-type devices C1-C4. FF

PCE

Rs

Rsh

Voc

Jsc

Ω·cm2

Ω·cm2

(V)

(mA/cm2)

PFS/BHJ/PFS

7.37

2.1k

0.871

16.9

0.65

9.54

C2

PFSF/BHJ/PFSF

97.6

1.01k

0.836

16.2

0.44

5.89

C3

PFB/BHJ/PFB

7.25k

271

0.768

14.0

0.17

1.80

C4

PFS/BHJ/PFSF

7.04

1.9k

0.893

16.9

0.67

10.2

Num.

structure

C1

23


(%)


4. CONCLUSIONS In conclusion, we synthesized a series of CPs containing the sulfonic acid fluorene unit, namely PFS, PFSF and PFB. The energy levels of the CPs can be readily tuned by copolymerizing the propyl sulfonic acid-functionalized fluorene with different units. The WF of ITO can be enhanced to 4.90, 5.10 and 4.70 eV after modifying with PFS, PFSF and PFB, respectively. PFS serves as AIL to reach a PCE of 11.0% in the fullerene-free OSC. PFSF and PFB can be used as CIL and the PFSF-modified OSC exhibits a PCE of 10.7%. More importantly, PFS shows excellent capacity in serving as the bifunctional interlayer. The fullerene-free OSC with symmetric structure, in which PFS is used as both the AIL and the CIL in the same device, is fabricated and a PCE of 9.54% is achieved. By using PFS as the AIL and PFSF as the CIL, a PCE of 10.2% is achieved in the fullerene-free OSC, indicating the diverse functions of sulfonic acid fluorene-based CPs as interlayers for modifying electrodes in fullerene-free OSCs. This work not only develops a series of interfacial layer materials but also demonstrates a promising application of the sulfonic acid-functionalized fluorene unit in developing superior and bifunctional CP interlayers for fullerene-free OSCs.

24


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ASSOCIATED CONTENT Supporting Information Chemical structures of photoactive layer materials for J52-2F, PBDB-T, PBDTTTEFT, IT-M, ITIC and IEICO-4F. The J-V curves, EQE and photovoltaic parameters of D,E, F-type devices using different photoactive layers. Electron Paramagnetic Resonance (EPR) curves of PFS, PFSF, and PFB. Calibration curves of film thickness to UV-vis absorption.

AUTHOR INFORMATION Corresponding Authors * To whom correspondence should be addressed: 1. Prof. Jianhui Hou, E-mail: [email protected]; Tel: +86-10-82615900; address: Zhongguancun North First Street 2, Beijing 100190, China 2. Prof. Bowei Xu, E-mail:[email protected]; Tel: +86-10-82362542; address: Zhongguancun North First Street 2, Beijing 100190, China

ORCID Jianhui, Hou: 0000-0002-2105-6922 Bowei Xu: 0000-0001-6467-6147 Notes 25



The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grants 51673201, 21504095) and the Chinese Academy of Sciences (Grant XDB12030200).

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Conjugated Polymers Containing Sulfonic Acid Fluorene Unit for

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